A South Korean government source with military knowledge told CNN that weapon was likely a piece of long-range artillery “likely to be a multiple rocket launcher.” At the time, South Korean Unification Ministry deputy spokeswoman Lee Eu-gene downplayed the significance of the 2018 event, saying Kim had been continuing his inspections in the military sector “intermittently.”

The downplay could reflect the actual state of affairs. But there is a worrying alternative, made possible by the participation of physicists from the former Soviet Union. In the past few years, the North’s nuke program, formerly glacial, has produced a torrent of results.

Quoting,

No missile launch was detected by US Northern Command and Strategic Command, according to US Department of Defense officials.

The absence of a missile launch detection, if not a very small missile, still allows a weapon like the nuclear Davy Crockett M-28, which resembles an upsized Panzerfaust antitank weapon from World War II. The recoilless rifle design consists of a launcher tube containing a powder charge that acts against a stick within the barrel. At the forward end of the stick is a bulb that contains the destructive device. The Panzerfaust contained a conventional shaped charge. The Davy Crockett contained a version of the W54 nuclear warhead.

Is Kim inviting us to fill in the blank after “tactical” with “nuclear”? An assessment could center on the shape of the plutonium pit in the “primary” of a warhead. Two practical choices exist: round, or like a football (elliptical)/ linear implosion. Round is most efficient, and requires the least fissile material. Elliptical/ linear implosion is the choice of space limited applications, like a skinny MIRV warhead, a nuclear artillery shell, or a Davy Crockett.

In flight, the Davy Crockett is a skinny stick with a bulb on the front. The fatter the bulb, the less aerodynamic it is. The greater yield of the round design is of no value with very short range, as some distance is required to survive even a very small nuclear explosion.

Although the U.S. makes wide use of linear implosion primaries with elliptical pits, a publicity photo of a happy Kim with a North Korean “physics package” goes against this design. Missile warheads are more sensitive to weight and yield than to shape. Linear implosion requires more fissile material, with greater weight for a given yield than a “round” design.

Should we look for evidence of linear implosion to rule in/out a battlefield weapon of Davy Crockett ilk? This risks captivation by the U.S. approach to weapons design, which aims for best-in-class. North Korea’s iffy rockets dispose of this.